Field of the Invention
The invention resides in the field of semiconductor technology and relates, more specifically, to a process for structuring a metal-containing layer.
In semiconductor technology, there is a high demand for structuring processes for a wide range of layers made from different materials. For example, for the fabrication of CMOS transistors, it is necessary to etch the relatively thin gate dielectric without contaminating adjacent regions. Hitherto, silicon oxide has been used as dielectric material. However, the increasing trend toward miniaturization means that the gate dielectric has to become increasingly thin in order to maintain the required capacitive coupling of the gate electrode to the channel region situated beneath the gate dielectric. However, reducing the thickness of the gate dielectric entails the risk of an undesirable electrical breakdown and the possibility of tunneling of electrons. Therefore, it is attempted to replace the silicon oxide with other materials of a higher dielectric constant which, on account of their dielectric constant, allow a greater material thickness for the same electrical properties (capacitance).
Furthermore, it is also desired, during the fabrication of semiconductor memories which use a capacitor for storing charges (DRAM), for the capacitor dielectric made from silicon oxide which has previously been used to be replaced by a material with a higher dielectric constant.
The structuring processes used are, for example, physical processes (sputtering), in which accelerated ions are directed onto the layer to be structured and thus abrade this layer. A problem of these processes is the relatively low selectivity between the layer to be structured and the mask material used, so that accurate transfer of the structure is often impossible.
By contract, processes which use etching gases have a higher selectivity. Examples of processes of that type are described, for example, in U.S. Pat. No. 3,951,709, German published patent application DE 27 38 839 A1, Japanese published patent application JP 07-254606 A, and U.S. Pat. Nos. 4,026,742, 5,382,320, and 3,923,568. Those prior art processes, however, can only partially be applied to layers of dielectric nature.
The object of the present invention is to provide a method of structuring a metal-containing layer which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this general kind, and which enables simple structuring of a metal-containing layer, in particular a dielectric layer, with a high selectivity.
With the above and other objects in view there is provided, in accordance with the invention, a method of structuring a metal-containing layer, which comprises the following steps:
providing a metal-containing layer on a substrate, the metal-containing layer consisting of titanium (Ti), titanium oxide (TiOx), tantalum (Ta), tantalum oxide (TaxOy), aluminum (Al), aluminum oxide (AlxOy) or a mixture of these metals and oxides;
applying and structuring an etching mask on the metal-containing layer; and
etching the metal-containing layer, using the etching mask, in a plasma-assisted etching gas atmosphere at a temperature of over 130xc2x0 C. and in a presence of at least one halogen compound and at least one oxidizing agent, wherein a concentration of the oxidizing agent is higher than a concentration of the halogen compound, and the halogen compound contains NF3.
In accordance with an added feature of the invention, the halogen compound is a fluorinated carbon or a fluorinated hydrocarbon.
In accordance with an additional feature of the invention, the halogen compound is SF6 or CF4.
In accordance with another feature of the invention, the halogen compound is CHF3.
In accordance with a further feature of the invention, the temperature during the etching is set to above 200xc2x0 C., and preferably to about 250xc2x0 C.
In accordance with again an added feature of the invention, the halogen compound is present in a concentration by volume of between 1% and 10%, and the oxidizing agent is present in a concentration by volume of between 90% and 99%.
In accordance with again an additional feature of the invention, the etching mask is formed of a material which, by reaction with the oxidizing agent, can be to a very great extent passivated with respect to the halogen compound. Preferably, the etching mask is formed of silicon oxide or polysilicon.
In accordance with again another feature of the invention, oxygen is employed as the oxidizing agent.
In accordance with a concomitant feature of the invention, the etching of the metal-containing layer is assisted by additionally employing physical abrasion with ions.
The process according to the invention can be used to etch metal-containing layers in particular with a high selectivity with respect to silicon oxide and polysilicon. The metal-containing layer may in this case consist of titanium (Ti), tantalum (Ta), titanium oxide (TiOx), tantalum oxide (TaxOy), aluminum (Al), aluminum oxide (AlxOy) or a mixture of the above-mentioned oxides and metals. It has been found that metal-containing layers of this type, at temperatures of over 130xc2x0 C., react with the halogen radicals which are produced in the plasma and form volatile compounds. These can therefore readily be vaporized from the metal-containing layer. It is expedient for the temperature for etching of the metal-containing layer to be selected between 200xc2x0 C. and 300xc2x0 C., preferably at approximately 250xc2x0 C. At higher temperatures, the metal halides formed during the etching can be vaporized more easily, so that if appropriate it is even possible to select temperatures of over 300xc2x0 C., for example of 350xc2x0 C. This is particularly advantageous for metal halides of low volatility.
At high temperatures of this level, it is recommended to use an etching mask made from silicon oxide or polysilicon. To prevent these mask materials from being attacked by the halogen compound, a relatively high proportion of oxidizing agent is added to the etching gas mixture. This oxidizing agent passivates at least the exposed mask surface, i.e. the upper side and the side edges of the etching mask, by oxidation, so that the halogen compound can only attack the etching mask to a slight extent. As a result, the etching mask is passivated to a very great extent. This also contributes to a high dimensional stability during the transfer of the structure from the etching mask to the metal-containing layer. A suitable oxidizing agent is in particular oxygen in a concentration of over 50%, preferably between 90% and 99%. Preferred halogen compounds are fluorinated carbons or hydrocarbons, in particular CF4, SF6, NF3 or CHF3, the concentration of which is lower than the concentration of the oxidizing agent. The concentration of the halogen compound is preferably below 20%, particularly preferably between 1% and 10%.
When using fluorinated hydrocarbons, a polymer film may form during the etching process.
Under certain circumstances, this film may then have to be removed. It is also possible to leave this polymer film on the layer which is to be etched, should this prove desirable.
In the case of fluorinated carbons, a polymer film of this type is not formed, so that particularly clean etching can be carried out using these etching gases.
The relatively high proportion of oxidizing agent during the etching can also lead to oxidation of the metal-containing layer, which is consequently easier for the halogen compound to attack. It is even possible, by means of the process according to the invention, to etch precious metals or metal oxide layers which are partially in polycrystalline form.
This process is preferably used for the fabrication of transistors and of memory cells.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method of structuring a metal-containing layer, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.